Process for producing high-strength seamless steel pipe having excellent sulfide stress cracking resistance

Abstract

PCT No. PCT/JP96/01274 Sec. 371 Date Feb. 5, 1998 Sec. 102(e) Date Feb. 5, 1998 PCT Filed May 15, 1996 PCT Pub. No. WO96/36742 PCT Pub. Date Nov. 21, 1996A process for producing a seamless steel pipe wherein pipe manufacturing steps and the heat treatment steps are carried out in one production line. The properties of the pipe are comparative or superior to those of the pipe manufactured in the conventional reheating, quenching and tempering process. The process is characterized by using the billet of a low alloy steel containing C: 0.15-0.50%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, Al: 0.005-0.50%, Ti: 0.005-0.50% and Nb: 0.003-0.50%, and comprising the following steps (1) to (5). (1) hot rolling with 40% or more of cross sectional reduction ratio, (2) finishing the hot rolling in a temperature range of 800-1100 DEG C., (3) putting the manufactured steel pipe promptly in a complementary heating apparatus after the finish rolling, and complementarily heating at the temperature and time satisfying the following formula (a). (4) quenching the steel pipe immediately after taking out of the complementary heating apparatus, and (5) tempering the pipe at a temperature not higher than the Ac1 transformation point as the last heat treatment.23500</=(T+273)x(21+log t)</=26000(a)where, T ( DEG C.) is a temperature of not lower than 850 DEG C., and t is time (hr). Further, an intermediate heat treatment consisting of quenching or combination of quenching and tempering may be applied between the steps (4) and (5).

We claim: 
     
       1. A process comprising steps of hot piercing and hot rolling for producing a high strength seamless steel pipe, having excellent sulfide stress cracking resistance, characterized by using a billet of low alloy steel which contains, in weight %, 0.15-0.50% of C, 0.1-1-1.5% of Cr, 0.1-1.5% of Mo, 0.005-0.50% of Al, 0.005-0.50% of Ti and 0.003-0.50% of Nb, and comprising the followings steps: (1) hot piercing the billet into a hollow shell,   (2) hot rolling the hollow shell with 40% or more of cross sectional reduction ratio,   (2) the hollow shell   (3) finishing the hot rolling in a temperature range of 800-1100° C.,   (4) putting the manufactured steel pipe promptly in a complementary heating apparatus after the finish rolling, and complementarity heating at the temperature and time satisfying the following formula (a),   (5) quenching the steel pipe immediately after taking out of the complementary heating apparatus, and   (6) tempering the pipe at a temperature not higher than the Ac 1  transformation point as the last heat treatment,   23500≦(T+273)×(21+log t)≦26000         (a)     where, T (°C.) is a temperature of not lower than 850° C., and t is a time (hr).     
     
     
       2. A process for producing a high strength seamless steel pipe, having excellent sulfide stress cracking resistance according to claim 1, characterized by further comprising one or more times intermediate heat treating which consists of quenching or combination of quenching and tempering, between the above-mentioned quenching step (5) and the last heat treatment step (6). 
     
     
       3. A process for producing a high strength seamless steel pipe, having excellent sulfide stress cracking resistance according to claim 1, characterized by using the steel billet which consists essentially of, in weight %, 0.15-0.50% of C, up to 1.5% Mn, 0.1-1.5% of Cr, 0.1-1.5% of Mo, 0.005-0.50% of Al, 0.005-0.50% of Ti, 0.003-0.50% of Nb, up to 0.010% of N, up to 0.01% of O, up to 0.05% of P, up to 0.01% of S, up to 0.1% of Ni, up to 0.5% of V, up to 0.5% of Zr, up to 0.01% of B, up to 0.01% of Ca, up to 2.0% of W, and the balance of Fe and incidental impurities, and each amount of Ti, Zr and N is defined by the following formula (b).   Ti(%)-(48/14)×{N(%)-(14/91)×Zr(%)}≧0    (b)     
     
     
       4. A process for producing a high strength seamless steel pipe, having excellent sulfide stress cracking resistance according to claim 3, wherein the steel billet further contains 0.05-0.5 weight % of V. 
     
     
       5. A process for producing a high strength seamless steel pipe having, excellent sulfide stress cracking resistance according to claim 3 or claim 4, using the steel billet in which Si content or Mn content is not more than 0.1 weight % respectively, or both of Si content and Mn content are not more than 0.1 weight %. 
     
     
       6. A process for producing a high strength seamless steel pipe, having excellent sulfide stress cracking resistance according to claim 3, or claim 4, using the steel billet in which P as an impurity is not more than 0.005 weight %, or S as an impurity is not more than 0.0007 weight %, or P as an impurity is not more than 0.005 weight % and S as an impurity is not more than 0.0007 weight %. 
     
     
       7. A process for producing a high strength seamless steel pipe, having excellent sulfide stress cracking resistance according to claim 5, using the steel billet in which P as an impurity is not more than 0.005 weight %, or S as an impurity is not more than 0.0007 weight %, or P as an impurity is not more than 0.005 weight % and S as an impurity is not more than 0.0007 weight %.